The two reasons why port1 can be shut down are loop guard protection and Spanning Tree Protocol (STP).
Loop guard protection: This is a feature that helps to prevent switching loops in a network.expand_more A loop guard can be configured on a port to monitor for specific traffic patterns that indicate a loop. If loop guard protection detects a loop, it will shut down the port to prevent the loop from causing problems.
STP: STP is a protocol that helps to prevent switching loops.expand_more When multiple paths exist between two network devices, STP will block all but one of the paths, creating a loop-free topology.expand_more If STP detects a loop, it will shut down the ports that are involved in the loop.
In the exhibit, both ports 1 and 2 are configured with the same native VLAN 10. This configuration could create a switching loop if both ports are connected to devices on the same network segment. If a loop occurs, loop guard protection or STP could shut down port1 to prevent the loop from causing problems.
Reference:
Fortinet FortiSwitch 7.2 Administration Guide https://docs.fortinet.com/document/fortigate/7.4.3/administration-guide/954635/getting-started